In a uv curable sealant, the uv radiation causes release of initiators in the sealant, leading to cross-linking or curing of the sealant (e.g., an epoxy). Often there is a problem of accessing regions of sealant with the uv radiation when there are metallized sections through which the uv cannot penetrate. For example, when the uv light is incident normal to the plane of the sealant, only that part of the sealant not shadowed by metallization can be expected to be affected by the uv radiation. As a result, sealant residing beneath metallization generally remains uncured. If there are alternating lines of metallization, only the sealant not covered by the metallization becomes cured, resulting in alternating sections of cured and uncured polymer. Such configurations (and hence problems) exist along boundaries of a flat panel display, where input/output lines of the panel block the uv irradiation used to cure the sealant between the two glass panels forming the display.
Currently, glass substrates used in a flat panel display are affixed to each other with a thermal setting epoxy, prior to filling of the panel with the liquid crystal material. This method of construction makes it possible to use relatively high curing temperatures, since the liquid crystal is not exposed to these temperatures (which the liquid crystal cannot tolerate). This thermal sealing method provides a very robust seal.
In general, a narrow filament of sealant material is dispensed over the entire outer periphery of one of the two substrates forming the panel except for a small region left vacant for liquid crystal filling. The two substrates are then joined and the composite baked to cure the sealant. After filling with liquid crystal, the small fill-opening is sealed with a uv epoxy, and cured or cross-linked by radiation from one or more uv lamps directed in a direction parallel to the panel.
A newly proposed scheme, based on the disclosure of U.S. Pat. No. 5,263,888 to Ishihara et al., uses a one drop fill method, or ODF. The liquid crystal is deposited onto one of the substrates prior to affixing the second substrate. A uv sealant is then used over the entire periphery to hold the first substrate to the second substrate, and the sealant cured with uv radiation from one or more mercury lamps. The problem with this procedure is that there are then regions of metallization on both substrates that prevent uv light from reaching sealant using normal incidence uv illumination.
One previously proposed exposure method for overcoming this problem involves use of edge-on illumination in conjunction with a high intensity uv light source such as a laser. However this method can cause damage to the liquid crystal should the uv penetrate the lateral dimension of the glue seal, and enter into the region of the liquid crystal. It has also been suggested that a barrier wall could alleviate this potential problem, but it is not clear that panel manufacturers are willing to adopt this proposed scheme as it adds cost to the manufacturing cycle.
Accordingly, it is an object of the invention to provide an improved method for affixing two objects together with a curable sealant, preferably uv, but other wavelength sealants are acceptable.
It is a further object of the invention to provide a method and system for curing such a uv-curable sealant that is shadowed by metallization components.